Regarding cellulose esters, cellulose acetate is used in such applications as tobacco filter, film, etc. while cellulose acetate butyrate and cellulose nitrate are used in lacquers and other applications. Meanwhile, cellulose esters are decomposed or degraded by ultraviolet light, heat, air and outdoor conditions in general but their decomposition (degradation) rates are extremely low as compared with cellulose. Therefore, when a used article made of cellulose ester is discarded outdoors, it retains its shape for a long time to become a cause of environmental pollution. Particularly when high-consumption articles such as cigaret filters are discarded outdoors, the risk of pollution is very great. Moreover, such articles discarded outdoors can hardly be completely recovered and would entail an almost prohibitive cost of recovery. Disposal of articles by incineration involves large outputs of combustion heat which detract considerably from the serviceable life of the incinerator.
Unlike the ordinary synthetic polymers, cellulose esters as such are not easily moldable so that plasticizers are generally included in the molding compounds. For example, Japanese Patent Publication No. 16305/1968 discloses a cellulose acetate molding compound containing a polyester having an average molecular weight of 700 to 4000 as synthesized from a glycol and an aliphatic dibasic acid as a polymer plasticizer. This plasticizer is reportedly well compatible with cellulose acetate having a degree of acetylation not less than 52% (degree of substitution not less than 2.2).
Japanese Patent Application Laid open No. 276836/1986 also discloses a cellulose derivative resin composition containing a phthalic acid-based polyester with an average molecular weight of about 250-3000 for improved plasticity, non-volatility and non-migration characteristics. In this composition, too, a cellulose ester having a degree of acetylation not less than 52.2% (degree of substitution not less than 2.2) is employed.
Japanese Patent Publication No. 36015/1986 further discloses a molding composition for improved resistance to thermal degradation or deformation and improved processability of moldings, which composition comprises 10 to 25 parts by weight of a plasticizer relative to 100 parts by weight of a cellulose acetate having a degree of acetylation in the range of 50 to 57% (degree of substitution 2.1 to 2.5) and a degree of polymerization in the range of 50 to 120.
U.S. Pat. No. 3,781,381 discloses a composition comprising an .epsilon.-caprolactone or other cyclic ester polymer as a modifier and a cellulose ester or other thermoplastic resin. This literature mentions a cellulose acetate with a degree of substitution equal to 2.5 (Example 14).
Furthermore, U.S. Pat. No. 4,731,122 discloses a thermoplastic resin molding composition comprising 35 to 70% of cellulose acetate butyrate, 10 to 40% of cellulose acetate, 15 to 30% of tributyl citrate, 5 to 30% of triethyl citrate and 1 to 10% of polyethylene glycol. The cellulose acetate butyrate used there has a butyryl group content of 17% and an acetyl group content of 29.5% (degree of substitution about 2.5) and the cellulose acetate used has an acetyl group content of 39.8% (degree of substitution about 2.5).
However, none of these known compositions reflect a consideration of biodegradability and actually all are lacking in biodegradability.
As a biodegradable resin composition containing a cellulose ester, Japanese Patent Application Laid open No. 142344/1992 proposes a biodegradable synthetic resin composition comprising 50 to 90 weight % of a cellulose ester, 0 to 40 weight % of a plasticizer and 5 to 30 weight % of an aliphatic polyester with an average molecular weight of 500 to 3000.
However, the biodegradability of this composition is largely dependent on the biodegradability of the plasticizer and aliphatic polyester added and the biodegradability of the cellulose ester as such is not remarkable. Moreover, because a citric acid ester or the like has to be used as the plasticizer, it is impossible to impart high moldability to the cellulose ester.